Torque converters and dynamic dampers which have the purpose of dampening vibrations and limit the maximum torque in the drill string are known from the oil industry, among other things. In a drill string several thousand metres long, vibrations may occur due to the varying torque on the string. Such vibrations are most often caused by the drill bit hitting different rock formations, which involves that a varying load is applied to the drill string. Then, when the drill string is rotated by the torque from the drilling machine, great amounts of energy are stored, which are released when the drill bit suddenly breaks loose.
Uncontrolled release of energy leads to extreme stresses on the drill string and drill bit. Today's controllable drilling systems are provided with much electromechanical equipment, which may get damaged by extreme shocks and vibrations.
In the present invention the aim is to employ a device, which yields immediately and, by and by, seeks to pull the drill bit axially from the bottom whenever the torque exceeds a predetermined value.
Such a device is particularly well suited for modern drill bits with fixed cutters, in which industrially manufactured diamonds of high hardness are used, but in which the structure is brittle and sensitive to impacts.
Within the oil industry problems connected with overloading of the drill string are well known, and there has been worked continuously on different solutions to remedy the problems. In this connection, reference is made to the following patents.                NO 315209        GB 5,083,623        U.S. Pat. No. 4,281,726        GP 065 601 A1        U.S. Pat. No. 4,443,206        WO 97/01693        